1. Field of the Invention
The present invention relates generally to a lithographic apparatus. More specifically, the present invention relates to an article support of a lithographic apparatus.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning means, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the conventional lithographic projection apparatus, during lithographic processes, an article, such as a wafer or reticle, is clamped on an article support by a clamping force, that may range from vacuum pressure forces, electrostatic forces, intermolecular binding forces or just gravity force. In the context of this application, the “article” may be any of the above mentioned terms of wafer, reticle, mask, or substrate, more specifically terms such as a substrate to be processed in manufacturing devices employing lithographic projection techniques; or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
European patent application EP0947884 describes a lithographic apparatus having an article holder wherein protrusions are arranged to improve the flatness of the article. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other and thereby form a bed of supporting members that support the article. However, such a configuration is costly to manufacture, since the protrusions need to be perfectly level. In this respect, it is desirable to reduce the number of protrusions of the article. However, when reducing the number of protrusions, the article tends to be supported more unevenly, which may result in image degradation and loss of resolution. A further problem is that in most cases, the article is not perfectly level, so that leveling thereof requires a large support area that is perfectly level, and the use of a relatively high clamping pressure.